The present invention relates to the field of communications in general, and more particularly, to wireless communications.
In radiotelephone and other wireless communication systems, wireless terminals often acquire service by “locking onto” or “camping” on a Digital Control Channel (DCCH) after powering up and/or when transitioning between base stations to provide, for example, synchronization or handshaking with the wireless communications system. In some situations, such as power-up, the wireless terminal may not know which control channel to camp on. Thus, the wireless terminal may have to locate such a control channel. This may be accomplished by “scanning” channels in the wireless communications system to locate a control channel which is considered acceptable. The process of scanning channels to locate such a control channel is sometimes referred to as an Intelligent Roaming (“IR”) search or as a Power-Up Scan.
In some wireless communications systems, the IR search is performed as a series of separate scans of selected operating frequencies. For instance, the system standard for TIA/EIA-136 wireless communications systems specifies an IR search that may include as many as four separate stages. In the first stage of the IR search, the wireless terminal scans a set of Private Operating Frequencies (“POF”). These private operating frequencies are the frequencies on which certain private or residential systems operate (at least in some geographic areas) to which the wireless terminal is allowed access. If a DCCH that is part of such a private or residential system having an acceptable received signal strength is identified in the POF scan, then the wireless terminal may camp on the DCCH.
If a private or residential system DCCH is not found during the POF scan, the wireless terminal may proceed to the second stage of the IR search. In this stage, control channels maintained in memory by the wireless terminal in a Digital control channel History Table (DHT) are scanned. The DHT contains a non-redundant listing of the frequency band and DCCH channel number of the control channels (excluding control channels on POF's) on which the wireless terminal has most recently camped. The DHT can contain multiple entries for a particular frequency band. The DHT is provided in TIA/EIA-136 compatible wireless terminals to speed-up the search for a control channel during an IR search, as, in many cases, an acceptable control channel corresponding to a highest priority service provider will be listed in the DHT. Under TIA/EIA-136, a control channel identified during the DHT scan is only considered acceptable for camping if it has at least the minimum received signal strength and if it is in a frequency band operated by either a Home or a Partner service provider.
If an acceptable control channel is not identified during the DHT scan, the IR search may continue to a third stage, which is referred to as the wideband scan. The wideband scan is typically the most involved of the channel scans. During the wideband scan, each of the frequency bands specified in the Intelligent Roaming Database (IRDB) is scanned, and the wireless terminal once again seeks to identify a DCCH that is on a frequency band operated by a Home or Partner service provider that has a minimum receive signal strength level. In lieu of scanning all of the hundreds of channels that are part of each frequency band, during the wideband scan each frequency band is typically divided into a number of “sub-bands” or “probability blocks”, and the channels having the highest received signal strength in each sub-band/probability block are selected for scanning.
During the wideband scan, the wireless terminal will examine the first control channel identified in each frequency band to determine if the frequency band is operated by a Home, Partner, Favored, Neutral or Forbidden service provider. If the frequency band is operated by a Favored, Neutral or Forbidden service provider, the scan of the band is typically aborted at that point, and if the frequency band is operated by a Favored or Neutral service provider, the frequency band, along with the highest received signal strength channels from each sub-band or probability block may be added to a “Re-Scan List” that identifies the frequency band channel pairs that are scanned during a fourth and final stage of the IR search.
If an acceptable control channel is not identified during the wideband scan, the IR search may proceed to the fourth or re-scan stage. During this stage, the wireless terminal scans each of the frequency band-channel pairs included in the Re-Scan List. During the fourth stage of the IR search, if the wireless terminal identifies a DCCH having at least the minimum received signal strength that is operated by a Favored or Neutral service provider, the wireless terminal attempts to camp on that DCCH.
The time needed to locate an acceptable control channel using the IR search described above may be considerable. For example, a scan of all bands may take about 5 minutes. Unfortunately, during the time when the wireless terminal is performing the IR search the wireless terminal may not be able to receive pages from the wireless communications system, possibly resulting in lost calls.
Some types of wireless terminals, such as those which use Code Division Multiple Access (CDMA) and Global System for Mobile Communications (GSM) type communications, may be capable of performing other operations while simultaneously performing the scanning operation described above. For example, CDMA and GSM compliant wireless terminals may be capable of performing Voice-Activated Dialing (VAD) operations while simultaneously camping on or scanning for a control channel. Other types of wireless terminals, such as those that use Time Division Multiple Access (TDMA) type communications, may not be capable of scanning or camping while simultaneously performing other operations. For example, TDMA wireless terminals may not be capable of performing VAD operations while simultaneously camping on a control channel.